It is proposed to isolate lymphoid cells with receptors for the carrier and haptenic determinants of antigen molecules. L-tyrosine- azobenzenearsonate served as a carrier for a poly-gamma-D-glutamic acid hapten of M.W. 35,000 in guinea pigs. Porcine glucagon, a polypeptide of 29 amino acids, was immunogenic in rabbits and guinea pigs. The antigenic determinants of glucagon were investigated with isolated tryptic peptides of the hormone. Antibodies elicited by glucogon had specificity for the N-terminal region, whereas only the C-terminal dodecapeptide was active in lymphocyte stimulation tests. Thus, resolution of the region on the glucagon molecule recognized by antigen- reactive cells from that against which antibody specificity is directed appears to have been effected. The minimum recognition determinant, the smallest C-terminal peptide which stimulates spleen cells from glucagon- immunized animals, will be synthesized. This peptide should serve as a carrier for a hapten, particularly in animals previously immunized with glucagon. It will also be determined if the minimum recognition peptide can itself elicit cellular immune responses. These systems will be applied to mice, since inbred strains are readily available and antisera specific for a surface antigen, theta, on thymic lymphocytes can be prepared. It is proposed to isolate lymphoid cells with surface receptors specific for L-tyr-azobenzenearsonate, poly-gamma-D-glu, and the N- and C-terminal sequences of glucagon, using polyacrylamide bead immunoadsorbent columns, and to assess the presence of the theta marker on the populations of cells. The ultimate goal of the project will be the isolation and comparison of antigen receptor molecules from the two cell populations.